This invention relates to an apparatus and method to assess cardiac function in a subject.
Non-invasive methods of determining cardiac functioning include the following:
a) Mechanical methods that include pulse recording of the jugular, carotid artery or apexcardiogram. This group also includes sound recordings, for example, use of the stethoscope and phonocardiographic techniques.
b) Electrical techniques which are best exemplified by the electrocardiogram (ECG).
c) Relatively more recent techniques include imaging techniques, for example echocardiography, nuclear cardiography, radiographic techniques and magnetic resonance imaging (MRI).
All of the above the mechanical methods, which rely on vibration and sound recording, involve measuring the movements of the body resulting from cardiac activity. This means that the mass of the body is part of the recording means. This is not desirable. Chest movements, for example, are dependent upon chest shape, and sound recording is dependent upon the amount of fat and the condition of the lung tissue for its amplitude. An accurate trace pattern is difficult to achieve and these techniques are therefore of limited diagnostic value.
Electrical methods measure only the electrical field generated by the heart. This cannot provide a direct measure of the cardiac forces generated by the heart and therefore these methods are incapable of evaluating the heart""s function as a pump.
Imaging techniques have limited ability to evaluate the force of the heart""s contraction.
Thus none of the above methods is capable of measuring the force of the heart""s contraction. As a result the evaluation of the condition of the myocardium is not possible. Heart attack risk cannot be determined by any known non-invasive method. A patient may be diagnosed as normal and yet die of a heart attack shortly after the diagnosis.
Relevant literature includes the following text books: Clinical Phonocardiography and External Pulse recording by Morton E. Tavel, 1978 Yearbook, Medical Publishing Inc.; Non-Invasive Diagnostic Techniques in Cardiology by Alberto Benchimol, 1977, The Williams and Wilkins Co.; and Cardiovascular Dynamics by Robert F. Rushmer, 1961, W.B. Saunders Company.
Rushmer first postulated that acceleration and deceleration of the various structures of the heart and blood explain heart sounds as well as their modifications with changing dynamic conditions. As acceleration is a function of force, the aortic blood acceleration is a manifestation of the force that sets the cardiac structures in motion. Other forces originate from the pressure gradient between the aorta and the left ventricle, which acts over the closed semilunar valve. The valve behaves like a circular, stretched membrane in which the thin, flexible leaflets can be stretched in all directions by the differential aortaxe2x80x94ventricular pressure. The energy of the rapid ejection phase of the left ventricle expands the aorta and the stored energy is in direct relationship to its wall elasticity. Measurement of the amplitude of the wave created after the maximum ejection rate, is a measure of the elasticity of the wall of the aorta. The elasticity of the aortic valve can also be measured by measuring the amplitude of the wave created after the valve is closed. The most sensitive indicators of performance are the rates of change of momentum as indicated by changes in velocity of the blood and heart mass. This acceleration is directly indicative of myocardial contractility which is one of the most difficult parameters to measure. In 1964 Rushmer established a direct relationship between the initial ventricular impulse and the peak flow acceleration during the systolic ejectionxe2x80x94see Circulationxe2x80x94Volume 29: 268-283 1964.
Commonly owned U.S. Pat. No. 5,865,759 discloses a method and apparatus for measuring cardiac function using an external sensor positioned against the thyroid cartilage in the neck. The subject matter of U.S. Pat. No. 5,865,759 is incorporated herein by reference. The sensor detects the response of the thyroid cartilage to heart function and generates a signal that is fed to a signal processing unit to generate a waveform signal characteristic of heart function for assessment by a user. The apparatus and method of U.S. Pat. No. 5,865,759 provide reliable, accurate and inexpensive assessment of cardiac function.
The present invention provides an improved apparatus and method for assessment of cardiac activity by directly measuring the movement of the trachea. The apparatus and method rely on introduction of a sensing apparatus into the throat of the user to engage with the trachea. This arrangement is sensitive to very small movement forces and permits accurate measurement of cardiac function with even finer details.
Accordingly, the present invention provides apparatus to assess cardiac function in a subject comprising:
a probe insertable and supportable in the trachea to transmit movement of the trachea in response to cardiac function through the probe;
a sensor to detect the transmitted movement of the trachea and generate a signal indicative of the movement of the trachea; and
a signal processing unit to receive the signal from the sensor and generate a waveform signal characteristic of the cardiac function.
The probe can be a hollow tube having an internal passage to deliver air to the subject and whereby movement of the hollow tube serves to transmit the movement of the trachea.
Alternatively, the apparatus can employ an endotracheal tube for housing the probe in which case the probe comprises a tubular member having an inner end adapted to protrude from the endotracheal tube and engage against the carina region at which the trachea bifurcates into the lungs.
The present invention also provides apparatus to assess cardiac function in a subject comprising:
a tube insertable into the mouth of a subject such that a first end extends into the trachea and a second end protrudes from the mouth;
a flexible support extendable from the tube to engage the trachea and suspend the tube within the trachea for movement of the tube along the longitudinal axis of the tube;
a rigid anchor extendable from the tube to engage the trachea and transmit movement of the trachea due to cardiac function to the tube;
a sensor to sense the movement of the tube and generate a signal indicative of the movement of the trachea; and
a signal processing unit to receive the signal from the sensor and generate a waveform signal characteristic of the cardiac function.
In a still further aspect, the present invention provides a method of assessing cardiac function in a subject comprising:
supporting a probe in the trachea to transmit movement of the trachea in response to cardiac function;
sensing the movement transmitted by the probe;
generating and displaying a waveform signal based on movement transmitted by the probe; and
assessing the waveform signal to determine cardiac function.
The apparatus and method of the present invention are intended to be used primarily with human patients, however, the subject matter also finds application with animal subjects. The apparatus and method can be used with a conscious subject or when the subject is anaesthetised, for example, during surgery.